The effect of two proteases associated with transformed cells upon the structure and composition of the plasma membranes of normal and transformed cells will be examined. Cultures of malignant but not normal cells produce a potent plasminogen activator. This protease is very efficient in converting the plasminogen found in serum to the active protease plasmin. Normal and transformed cells will be grown under conditions where the proteolytic activities of the two enzymes are eliminated either by the removal of plasminogen from the serum or by the inclusion of non- toxic protease inhibitors in the culture medium. The composition and structure of the plasma membranes of cells grown under these conditions will be analyzed by three techniques. Polyacrylamide gels of purified plasma membranes will be utilized to detect differences in protein composition. The distribution of the 80 A intramembranous particles as visualized by electron microscopy after freeze fracture of intact cells will be used to probe for differences in membrane structure. Finally, membrane architecture will be examined in whole cells by assaying the ability of a particular membrane protein, "tissue factor", to catalyze clot formation. Several parameters of the expression of the transformed cell phenotype such as saturation density, growth in soft agar, and wound migration will also be monitored under conditions of high or low protease activity. Thus modifications in cell phenotype will be correlated with changes in membrane structure.